Field Resistance of Coat Protein Transgenic Papaya to Papaya ringspot virus in Jamaica

In 1949, Jensen reported that the first papaya disease attributed to a virus was described in the Caribbean (14). According to Jensen, the virus, presumably Papaya ringspot virus (PRSV), was recorded by Smith in 1929 and was considered the primary reason for no large-scale cultivation of papaya (Carica papaya L.) in Jamaica. However, in the mid-1980s, papaya (also locally known as pawpaw) was promoted in the agricultural sector as a viable alternative to traditional crops. Subsequently, organized commercial orchards were established across the island with the Sunrise solo cultivar (12). Over 240 ha of papaya were put into production by 1994 (24), and Jamaica, by then, was recognized as a leading exporter of papaya in the Caribbean. With this rapid expansion in the cultivation of papaya, PRSV gained prominence and once again was recognized as a potential problem to the production of the fruit crop in Jamaica. PRSV, a member of the genus Potyvirus in the Potyviridae plant virus family, consists of flexuous particles with a monopartite RNA genome (5,20,27). In papaya, PRSV induces mosaic, leaf discolorations, severe leaf deformations, ringspot blemishes on fruits, and overall stunting of the plant (10). Over the past two decades, various gene transfer methodologies have been utilized in the improvement of papaya germ plasm for resistance or tolerance to PRSV. Both conventional methods involving the introgression of genes from relatives in Caricaceae (18) or genetic modification of commercial varieties with viral transgenes have been investigated (11). The latter approach of genetic engineering has proven very successful using the coat protein (CP) gene of the virus and has led to the development of PRSVresistant varieties in various regions (1,3,7,9,16,26). Field evaluations in Hawaii, Florida, and Taiwan have reported on resistance phenotypes ranging from moderate protection (3,26) to complete protection against systemic infections (8,17). Resistance conferred by CP transgenic papaya also has been described as sequence specific and associated with suppressed accumulation of CP transgene mRNA and protein expression (23). Most importantly, loss of vigor or horticultural characteristics of the transgenic cultivar has not been documented (8,11,17). Field tests of transgenic papaya in Hawaii report on percent soluble solids above the minimum required for commercial fruit and yields of almost three times those of industry averages (8,19). In Jamaica, transgenic papayas were developed through a technology transfer program with Cornell University and the Jamaica Agricultural Development Foundation. Papaya embryos were transformed with translatable (CPT) and nontranslatable (CPNT) versions of the CP gene of a virus isolate from one region of the island (along with nptII and uidA genes) via microprojectile bombardment (2,22). Varying levels of CP transcript and protein were detected in northern analysis and enzyme-linked immunosorbent assay (ELISA) with CPT lines, and as expected, CP expression was not detected in CPNT lines (22). All R0 plants, while maintained under greenhouse conditions, tested positive for the nptII gene in polymerase chain reaction (PCR) and expression protein in ELISA (22). Initial greenhouse evaluation identified lines resistant to mechanical inoculations with homologous and heterologous isolates of the virus (22). In this study, two generations (R0 and R1) of transgenic lines were field-tested in one of the traditional papaya producing regions of the island in order to assess resistance under natural virus pressure as well as horticultural performance of the lines.